Computer systems typically employ one or more interconnects to facilitate communication between system components, such as between processors and memory. Interconnects and/or expansion interfaces may also be used to support built-in and add on devices, such as IO (input/output) devices and expansion cards and the like. For many years after the personal computer was introduced, the primary form of interconnect was a parallel bus. Parallel bus structures were used for both internal data transfers and expansion buses, such as ISA (Industry Standard Architecture), MCA (Micro Channel Architecture), EISA (Extended industry Standard Architecture) and VESA Local Bus. In the early 1990's Intel Corporation introduced the PCI (Peripheral Component Interconnect) computer bus. PCI improved on earlier bus technologies by not only increasing the bus speed, but also introducing automatic configuration and transaction-based data transfers using shared address and data lines.
As time progressed, computer processor clock rates where increasing at a faster pace than parallel bus clock rates. As a result, computer workloads were often limited by interconnect bottlenecks rather than processor speed. Although parallel buses support the transfer of a large amount of data (e.g., 32 or even 64 bits under PCI-X) with each cycle, their clock rates are limited by timing skew considerations, leading to a practical limit to maximum bus speed. To overcome this problem, high-speed serial interconnects were developed. Examples of early serial interconnects include Serial ATA, USB (Universal Serial Bus), FireWire, and RapidIO.
Another standard serial interconnect that is widely used is PCI Express, also called PCIe, which was introduced in 2004 under the PCIe 1.0 standard. PCIe was designed to replace older PCI and PCI-X standards, while providing legacy support. PCIe employs point-to-point serial links rather than a shared parallel bus architecture. Each link supports a point-to-point, communication channel between two PCIe ports using one or more lanes, with each lane comprising a bi-directional serial link. The lanes are physically routed using a crossbar switch architecture, which supports communication between multiple devices at the same time. As a result of its inherent advantages, PCIe has replaced PCI as the most prevalent interconnect in today's personal computers. PCIe is an industry standard managed by the PCI-SIG (Special Interest Group). As such, PCIe pads are available from many ASIC and silicon vendors.
Recently, Intel introduced the QuickPath Interconnect® (QPI). QPI was initially implemented as a point-to-point processor interconnect replacing the Front Side Bus on platforms using high-performance processors, such as Intel® Xeon®, and Itanium® processors. QPI is scalable, and is particularly advantageous in systems having multiple processors employing shared memory resources. QPI transactions employ packet-based transfers using a multi-layer protocol architecture. Among its features is support for coherent transaction (e.g., memory coherency).
A significant amount of I/O bandwidth is consumed by memory transactions. One approach that is currently employed to increase memory transaction bandwidth is to employ a Fully Buffered DIMM (or FB-DIMM) architecture, which introduces an advanced memory buffer (AMB) between a memory controller and a memory module. Unlike the parallel bus architecture of traditional DRAMs, an FB-DIMM has a serial interface between the memory controller and the AMB. This enables an increase to the width of the memory without increasing the pin count of the memory controller beyond a feasible level. With this architecture, the memory controller does not write to the memory module directly; rather it is done via the AMB. The AMB can thus compensate for signal deterioration by buffering and resending the signal. In addition, the AMB can also offer error correction, without posing an overhead on the processor or the memory controller. In addition to BF-DIMM. Intel has recently introduced the Intel® Scalable Memory Interconnect (SMI) and Scalable Memory Buffers (SMB). The integrated SMI offers high-speed serial links to the SMBs, which support cost-effective, commodity RDDR3 memory.
Other recent advancements include multi-core processors and System on a Chip (SoC) architectures. Rather than interfacing discrete components on a printed circuit board or through use of other package configurations, on an SoC multiple components are integrated onto a single integrated chip. SoCs offer a number of advantages, including denser packaging, higher speed communication between functional components, and lower temperature operation. SoC designs also provide standardization, scalability, modularization, and reusability.
Although SoC architectures are clearly the future direction of system designs, there are instances where it is still advantageous to keep some components on separate chips or dies. For example, it may be advantageous to have components with dedicated or specialized functions, such as memory controllers, on separate chips. At the same time, it would be desirable to support data transfer rates with such external components as if they were integrated on the SoC.